CHAPTER X. GEOLOGY AND MINERALOGY OF THE CHANNEL ISLANDS. ANCIENT FORMATIONS. In no part of Europe, and in no group of islands readily accessible, are the physical geography and geology more closely related than in the Channel Islands. The tongues and spurs of porphyritic rock, of which almost all the promontories of the Atlantic coast of Europe consist, are well illustrated in Western Britany and in Cornwall, and not less so in the intervening rocky groups. Better able than most of the secondary rocks to resist the constant action of the sea, these rocks show themselves, partially denuded and generally rugged, in the neighbourhood of Cherbourg and Cape la Hague, but they appear in perfection in a succession of spurs pointing westward, forming the groups which together compose the islands we are considering. Exposed from their isolated position to the never ceasing beating of the waves during their elevation, these islands have been deprived of most of the covering of stratified rocks they may once have possessed, and except a patch here and there in the more sheltered positions, scarcely any indication exists of those deposited rocks that abound on all sides of them, and give variety to the geology of England and France. Noue of the old fossiliferous rocks appear, not a particle of stratified limestone of the secondary period is to be seen, and not a single patch of the old tertiaries that are so nobly represented on the opposite shores of Hampshire, and in the Isle of Wight, has ever been discovered. All is rude and bare and the mere skeleton of that rich and varied structure to which we are elsewhere accustomed; so that at first sight it may seem-as it has indeed been generally stated that the geologist has here little opportunity for investigation, and need only record the fact of the existence of such and such varieties of rock in certain localities. But the comparative anatomy and natural history of the earth, like that of its inhabitants, must be learnt from the skeleton. It is where the old, hard, nether-formed rocks frowningly appear above the water, where they are exposed in broad tabular masses laid bare by the waves, unclothed with soil, uncovered by sand and pebbles, that this history can best be learnt. It is there that natural sections of them may be studied, showing all varieties of early structural change, and the later action of water, and of constant change of temperature. Under such circumstances, when they form hills rather than mountains, and cliffs that may be approached rather than precipices that can be looked on only from a distance :—where they have been exposed to all varieties of change and disturbance-where they have been undermined and torn into shreds, but are still struggling with apparent success against their inevitable fate, that we read most of the great lessons they were intended to teach. For such reasons, and used in such a manner, the Channel Islands afford an admirable school for the geologist. The field is one which has not too many cultivators, but it will amply repay any amount of labour. There is, probably, no area of equal extent even in England, that presents more variety of detail, or from which more may be learnt. The points of interest, although numerous and important. NATURE OF THE ENQUIRY. 249 are concentrated at the two extremities of the geological scale. They involve inquiries concerning the mode of production and subsequent change of most varieties of crystalline rock and mineral veins, and thence they pass at once to the latest changes that have taken place upon the earth. On the one hand, we have numerous phenomena exemplifying the laws of composition and the original structure of the rocks called igneous, such as the syenites and porphyries, and other hypogene rocks. We learn the history of the stratified form of these rocks; the nature, extent, and cause of the various systematic clefts or fissures by which they are very generally intersected; the minerals and rocks by which the fissures have since become filled. We may learn, also, the nature of those modifications by which the rocks and systematic fissures or veins have been affected since their original formation, and the external or mechanical changes, whether of squeezing, elevation, depression, or weathering, either by sea wave or atmospheric exposure, to which they have been subjected. The consideration of questions of this kind is in itself of the highest importance in geology, and it leads to results of great practical value connected with the presence of metaliferous wealth, the applicability of stones for various purposes, and the origin and nature of soils and subsoils. There is another department of geology, in regard to which the Channel Islands are full of instruction. This relates, as we have stated above, to the last changes that have taken place upon the earth; it considers the disturbances and deposits whose date is historical rather than geological, and which result from causes still in operation at the same place. The deposit of peat, or of rolled pebbles and stratified sand; the removal of other similar deposits already bedded; the breaking through of barriers, more or less natural, and the introduction and operation of various forces, visible and calculable; the mode in which rocks are undermined, weathered, broken up, and carried off in No one can visit the rocky bays, and look into the innumerable pools of water left at half tide on every part of the shores of the Channel Islands, without seeing evidence of their unbounded wealth in zoophytes. Of these animals, the so-called sea anemones (actinia, anthea, bunodes, sagartia, Peachia, Edwardsia, are now so familiar from their frequent recurrence in every aquarium, that we need do no more than mention the fact of their presence. The more common species actinia mesembryanthemum and bunodes (tealia) crassicornis, abound on rocks left uncovered at half tide, and in caverns to which the sea has access. In such places they often so completely cover the rock as to exclude other growth. The deep, reddish colour of the |